International comparison CCRI(II)-S7 on the analysis of uncertainty budgets for 4πβγ coincidence counting

Detailed uncertainty reporting is imperative for proficiency tests and comparison exercises because uncertainties need to be comparable and trusted by all the participants. Even though participants do their best to follow the Guide to the Expression of Uncertainty in Measurement, ambiguities and divergences about uncertainty evaluation remain. Consequently, to analyze the situation, the CCRI(II) Uncertainties Working Group proposed a comparison exercise (CCRI(II)-S7) about the uncertainty evaluation of a relatively simple primary activity measurement: the standardization of a ⁶⁰Co source by coincidence counting. To be able to understand how various NMIs calculate coincidence counting uncertainties, our study focused on two of the dominant uncertainty components commonly quoted for 4πβγ coincidence counting in the International Reference System (SIR) submissions and Key Comparison exercises: efficiency-extrapolation and weighing. Participants from twelve different laboratories were sent the same set of measurement data from the analysis of a ⁶⁰Co solution standardized at the National Physical Laboratory (NPL). Our study demonstrated the extent of the different interpretations of the uncertainty components. Some factors causing large discrepancies were isolated and are discussed. Further studies of other techniques using a similar approach would be beneficial for the metrology community. (authors)